By your reckoning, all you want is a beginning and an end, no story. Most of a story is the filler. Take Lord of the Rings, Frodo gets sent on a mission, Frodo destroys the ring. Anything else that happens is filler material. So there is no story. Infact that was a bad example, Star Trek, has millions of fans, by your reckoning, most of that shouldn't exist. An episode may not be important to the overall main story, but its another story which become part of the universe and gives you a deeper understanding of that universe, the characters, the politics, sets the tone. - This is what makes it so popular and interesting to those people. So yes, it could be considered "filler" but without that, the story and universe that is huge would be tiny and very uninteresting.
I don't know a huge amount about the background to the wow lore, because there are other fictional universes that I find are far more indepth and are more too my liking. Pandaria is another story arc, that has clues and new information about existing lore aswell as expanding on some existing lore. You learn more about existing characters and races. Alot of the lore in Pandaland is interesting, new and quite exciting to a lot of people and they aren't just completely rehashing the same stuff they have used previously. There is so much to it that is new and interesting and without it would be a VERY dull and boring experience like many other story's out there, rather than its own thing.
Also, Lore - The backstory created around a fictional universe.
anyway, I'm tired and my point may not be clear.